Species Abundance and Diversity Chapter 16

Outline Introduction Species Abundance Lognormal Distribution Species Diversity Environmental Complexity Niches Disturbance and Diversity Intermediate Disturbance Hypothesis

Introduction Community: Association of interacting species inhabiting some defined area. Community Structure includes attributes such as number of species, relative species abundance, and species diversity. Guild: Group of organisms that all make their living in the same fashion. Seed eating animals in the desert. Life Form: Combination of structure and growth dynamics.

Species Abundance There are regularities in the relative abundance of species in communities that hold regardless of the ecosystem. Preston developed concept of distribution of commonness and rarity.

Lognormal Distribution Preston graphed abundance of species in collections as frequency distributions. Lognormal Distributions Bell-shaped curves. In most lognormal distributions, only portion of bell-shaped curve is apparent. Sample size has large effect. Significant effort to capture rare species.

Lognormal Distribution

Lognormal Distribution May proposed lognormal distribution is a statistical expectation. Sugihara suggested lognormal distribution is a consequence of the species within a community subdividing niche space.

Species Diversity Two factors define species diversity: Species Richness Number of species in the community. Species Evenness Relative abundance of species.

Species Diversity

Quantitative Index of Species Diversity Shannon Wiener Index: s H’ = -Σpi logepi i=l H’ = Value of SW diversity index. Pi = proportion of the ith species. Loge = natural logarithm of pi. S = Number of species in community.

Sum values across all species pi = 44/50 = 0.88, pi = 1/50 = 0.02 H’ example Community: S = 7 AAAAAAAAAA AAAAAAAAAA AAAAAAAAAA AAAAAAAAAA AAAABCDEFG Sum values across all species pi = 44/50 = 0.88, pi = 1/50 = 0.02 Calculate ln for each (ln 0.88 = - 0.128) Molles: Ecology 2nd Ed.

H’ Sum pi’s = 0.88 (-0.128) + 0.02 (- 3.91) +0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) + 0.02 (- 3.91) = - 0.58 Change sign; H’ = 0.58 Molles: Ecology 2nd Ed.

Another example: Community: S = 5 ABCDEABCDE ABCDEABCDE ABCDEABCDE ABCDEABCDE ABCDEABCDE H’ = 1.61 Molles: Ecology 2nd Ed.

What’s the purpose of H’? Includes both species richness and evenness Molles: Ecology 2nd Ed.

Rank Abundance Curves Can also portray relative abundance and species diversity within a community by plotting relative abundance of species against their rank in abundance. Greater evenness indicated by lower slope.

Rank Abundance Curves

Environmental Complexity In general, species diversity increases with environmental complexity or heterogeneity MacArthur found warbler diversity increased as vegetation stature increased Measured env. complexity as foliage height Many studies find positive relationship between env. complexity and species diversity Molles: Ecology 2nd Ed.

Environmental Complexity

Fig 16.9 Molles: Ecology 2nd Ed.

Diversity of Algae and Plants Hutchinson: Phytoplankton communities present paradox – they live in relatively simple environments and compete for the same nutrients, yet many species coexist without competitive exclusion Env. complexity may account for diversity Molles: Ecology 2nd Ed.

Diversity of Algae and Plants Algal niches appear to be defined by nutrient requirements Tilman (1977) found coexistence of freshwater diatoms depended upon ratio of silicate and phosphate Molles: Ecology 2nd Ed.

Tilman (1977) found conditions allowing coexistence Diatoms held different trophic niches Thus different diatoms would dominate different areas Trophic niches = nutrient requirements Molles: Ecology 2nd Ed.

Heterogeneity and Diversity of Tropical Forests Jordan – Amazon tropical forest diversity organized in two ways: 1. Large number of species live within most tropical forest communities Fig 16.14 Molles: Ecology 2nd Ed.

2. Large number of plant communities in a given area, each with distinctive species composition Fig 16.14 Molles: Ecology 2nd Ed.

Algal and Plant Species Diversity And Increased Nutrient Availability Repeatedly observed negative relationship between nutrient availability and algal and plant species diversity Adding nutrients to water or soils reduces diversity of plants and algae Reduces number of limiting nutrients Molles: Ecology 2nd Ed.

Disturbance and Diversity Disturbance difficult to define because it involves departure from “average conditions.” Average conditions may involve substantial variation Molles: Ecology 2nd Ed.

Sousa defined disturbance: Discrete, punctuated, killing, displacement, or damaging of one or more individuals that directly or indirectly creates an opportunity for new individuals to be established Molles: Ecology 2nd Ed.

Disturbance and Diversity Another definition of disturbance: Any relatively discrete event in time that disrupts ecosystem, community, or population structure and changes resources, substrate availability, or the physical environment Molles: Ecology 2nd Ed.

Two major characteristics of disturbance: Frequency Intensity Molles: Ecology 2nd Ed.

Intermediate Disturbance Hypothesis Connell disturbance is a prevalent feature that significantly influences community diversity Proposed that both high and low levels of disturbance reduce diversity Intermediate levels promote higher diversity Molles: Ecology 2nd Ed.

Sufficient time between disturbances allows wide variety of species to colonize, but not long enough to allow competitive exclusion Molles: Ecology 2nd Ed.

Disturbance and Diversity in The Intertidal Zone Sousa studied effects of disturbance on diversity of algae and invertebrates growing on boulders in the intertidal zone Predicted level of disturbance depends on boulder size Molles: Ecology 2nd Ed.

Large boulders move less = heavier Boulders with highest diversity of species had intermediate levels of disturbance Fig 16.18 Molles: Ecology 2nd Ed.

prairie dogs (Cynomys spp.) source of disturbance on N. A. prairies Disturbance and Diversity in Temperate Grasslands - burrowing as a disturbance Whicker and Detling prairie dogs (Cynomys spp.) source of disturbance on N. A. prairies Build extensive burrow systems Move 200-225 kg soil from underground to entrance Molles: Ecology 2nd Ed.

They removed vegetation around burrows Area opens to colonization Pest control programs reduced prairie dog populations 98% Eliminated dynamic influences on plant communities Molles: Ecology 2nd Ed.

Review Introduction Species Abundance Lognormal Distribution Species Diversity Environmental Complexity Niches Disturbance and Diversity Intermediate Disturbance Hypothesis